Shear beam load cell

ABSTRACT

A shear beam load cell comprising a cantilevered beam member (1) for measuring the shear force imposed on the beam member by the load (F) to be measured and one or more strain gauges mounted on the beam member so that they are affected by the deformation in the beam material. The free deflectable end portion (2) of the beam (1) has a section (10) with reduced material thickness, the upper side of this section being located substantially along the longitudinal neutral axis of the beam and defining a load application surface (11) for the load (F) to be measured. The load application surface (11) has a longitudinal extension, along the neutral axis or its extension, so that the point-of-load application for the load (F) to be measured can be moved along the load application surface in case of thermal expansions due to temperature variations in a tank, weighing container, foundation or the like.

FIELD OF THE INVENTION

The present invention relates to a load cell of the shear beam typeincluding a beam member for measuring the shear force imposed on thebeam member by the load to be measured and one or more strain gaugesmounted on the beam member so that they are affected by the deformationin the beam material.

BACKGROUND OF THE INVENTION

It is previously known to use beam-shaped load cells or forcetransducers based on resistive strain gauges in electronic weighingequipment of various types. The reason for this is that this type ofload cell has proved to be very reliable and they have also the abilityto maintain calibration and reproduction data for a long time and undersevere weighing conditions. The beam-shaped load cells are alsocomparatively small in size, which makes them suitable for built-in andlow-profile weighing applications. The load cells are usuallycylindrical and when they are installed they can therefore often replacean existing shaft or any other machine element in order to sense astrain or measure a load without requiring any modification of theequipment in which they are installed.

A significant quality of shear beam type load cells is the ability towithstand side forces, that is, forces acting in directions other thanin the measuring direction of the load cell. The load cells do notrequire any side supporting means, they can withstand side forces up to100-200% of the nominal load value.

However, a load cell can also be affected by other undesired forces andas is usual in measuring applications it is the temperature andspecifically temperature variations that cause measuring errors. Ofcourse the strain gauges which are used in the load cell have themselvesa certain temperature dependence caused by the fact that the measuringelement and the resistive wire have different thermal expansioncoefficients. If, however, at least four strain gauges are used and ifthey are of the same kind and glued on the same material, then thechanges in resistance caused by the temperature variations will be thesame for each of the strain gauges. As the strain gauges are arranged ina bridge circuit, the balance of the bridge will not be changed.

Undesired load forces affecting the measuring accuracy are also causedby, for instance, temperature expansions in the weighing container orplatform. For that reason the load cell is usually provided with abearing or other type of embodiment for eliminating these types ofdisturbing forces, such as side forces or twisting moments of torsion.

Swedish patent 82 01365-7 discloses a cylinder-shaped load cell withsuch a built-in bearing for permitting a side movement of the measuringbody on a plane base support member. The load cell can be positioneddirectly on a foundation or other base support member in such a mannerthat it is permitted to move in a side direction. By means of such abuilt-in bearing the load cell can be loaded via a loading bracketwithout any additional bearings required.

In many applications, however, these types of movements on a planefoundation or base support member are not permitted by the actual designof the weighing equipment. Furthermore, the bearing built into thecylinder-shaped measuring body makes this type of load-cellcomparatively expensive. The built-in bearing, in the form of, forinstance, slide bearing or a roll bearing, must be designed with a highdegree of accuracy and withstand the often very heavy load forces whichare applied to this type of load cell.

Another type of prior art load cell is disclosed in U.S. Pat. No.3,960,228. This load cell has a free deflectable beam secured to afoundation or other mounting means in such a manner that the freedeflectable weigh end of the beam is suitably supported for receiving avertical load force to be measured. The strain gauges are positioned onopposite sides of the beam and orientated substantially at a 45° angleto the longitudinal neutral axis of the beam. A recess is formed in thefree end of the deflectable beam in the load-measuring direction so thatthe vertical force of the load is applied on a load bearing surfacewithin the recess. The load bearing surface within the recess is locatedclose to the longitudinal neutral axis of the shear beam so that themoment arm and the twisting moment of an adverse side force affectingthe shear beam are minimized. Thus, in this type of load cell themeasurement inaccuracy, due to imperfect positioning of the straingauges on opposite sides of the shear beam, is reduced.

Also, in the case of a precise positioning of the two opposite straingauges, this arrangement does not solve the problems, however, due tothermal expansion in the weighing container or weighing platform. Theshear beam is secured to a foundation or the like and is not permittedany movement for compensating for such thermal expansions.

Swedish patent 311 573 discloses a load cell in the form of a shearbeam, in which the free deflectable end of the shear beam is fastened toone end of a second beam extending parallel to and freely at the side ofthe first beam. The second beam is shorter than the first beam and, thefree end of the second beam provides the receiving surface for the loadforce to be measured. This second beam is preferably made as a sleevemember freely enclosing the first beam.

One important advantage of such an arrangement is the fact that the loadcell is substantially insensitive to any change in point-of-loadapplication. Specifically, any changes of the point-of-load applicationdue to thermal expansion in a weighing container or a weighingfoundation affect the result of the measurement only to a very smalldegree.

Even if this type of load cell with a second sleeve enclosing the firstshear beam has this important advantage it should be understood thatthis type of load cell is more expensive in construction. Therefore, aload cell is desired which is more simple in construction.

SUMMARY OF THE INVENTION

The object of the present invention is to provide a shear beam load cellwhich is simpler in construction but which is very insensitive toundesired forces, such as adverse side forces and twisting moments aswell as thermal expansions.

According to the invention the free end of the shear beam has a sectionwith reduced material thickness, the upper surface of this section islocated substantially along the longitudinal neutral axis of the shearbeam and provides the surface upon which the load to be measured isapplied. This surface could be a plane surface, convex, concave or thelike, but the surface upon which the load is applied should have anextension in the longitudinal direction, that is, in the direction ofthe neutral axis, so that the point-of-load application could be movedalong the surface in case of thermal expansions.

Preferably, the load force is applied by means of a U-shaped loadingbracket which is supported on the surface and which is movable along thesurface in case of thermal expansions. In one preferred embodiment, theloading bracket is closed to provide at the same time a tilting stop.

The load application surface is preferably provided with a gliding stopoutermost on the beam to limit the movement along the surface for theload supporting bracket or the like.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following the invention will be described more in detail withreference to the accompanying drawings which are illustrating someexamples of the shear beam.

FIG. 1 is a perspective view of a load cell according to the presentinvention provided with a load supporting, closed bracket,

FIG. 2 illustrates some examples how the load application surface couldlook like and

FIG. 3 illustrates some examples of how the loading bracket can beconnected to the load application surface of the shear beam.

DETAILED DESCRIPTION OF THE INVENTION

The load cell illustrated in FIG. 1 generally comprises a beam member 1having a free deflectable end portion 2 and a mounting end portion 3which is in integral connection with a plate support section 4 fastenedto a foundation or the like by means of bolts 5, 6. Recesses 7 areformed on opposite sides of the free deflectable part of thecantilevered beam member for defining a symmetrical, substantiallyI-shaped transverse cross-section. Strain gauges are mounted in therecesses in a known manner for measuring the shear strain caused by aload F applied to the free deflectable end portion 2 of the beam bymeans of a loading bracket 8. In the example, the beam member 1 has thegeneral configuration of a parallelepiped, but it should be understoodthat the beam be cylindrical.

The mounting end portion 3 has a cable connection 9 for the straingauges. The strain gauge arrangement per se is conventional, however,and will not be described in any detail here.

From FIG. 1 it can be seen that the free deflectable end portion 2 ofthe beam has a section 10 with reduced material thickness. Specifically,on the upper side of the free deflectable beam portion material has beenmilled or cut away so that a load application surface 11 is formed,which surface is located along the longitudinal neutral axis N of thebeam. As already mentioned in the introductory portion of thespecification it is an advantage if the point-of-load application islocated on the neutral axis of the beam.

According to the present invention, the load application surface has alongitudinal extension, along the neutral axis of the beam or itsextension, so that the point-of-load application can be moved along thesurface. This is the case if the loading bracket 8 is connected to aload which is affected by thermal expansion. The loading bracket 8 willthen slide along the surface 11. The beam has a sliding or security stop12 to limit the magnitude of the longitudinal movement. The U-shapedloading bracket 8 is closed by means of a bottom member 13 so that theloading bracket also has the function of a tilting stop protection.

The load application surface has a somewhat rounded profile. In FIG. 2it is illustrated some examples with different profiles for the loadapplication surface. FIG. 2a shows a convex, somewhat rounded loadapplication surface, FIG. 2b a corresponding concave profile, FIG. 2c atrapezoidal, convex profile, FIG. 2d a corresponding concave profile andFIGS. 2e, 2f, and 2g illustrating the same surfaces but together withsliding stops 12', 12" and 12'".

As already mentioned the loading bracket is preferably closed so that italso can prevent a possible tipping motion. Its contact surface 14against the load application surface can be curved in the longitudinaldirection of the load application surface, as indicated in FIG. 3a, aswell as perpendicular to the longitudinal direction of the loadapplication surface, as indicated in FIG. 1. The curved contact surface14 in FIG. 3a reduces the contacting surface against the loadapplication surface and thereby also the friction, which is anadvantage. One alternative embodiment for reducing the friction isillustrated in FIG. 3b. In this case the loading bracket has a planecontact surface 14', seen in the longitudinal direction of the loadapplication surface, while the load application surface itself has asmall elevation 15 in its mid-section.

In certain cases it could be an advantage if the loading bracketmaintains a stable position on the midsection of the load applicationsurface of the mesuring beam. In FIG. 3c it is illustrated how the loadapplication surface in its longitudinal direction has been made somewhatconcave so that the loading bracket tends to be retained in a stableneutral position. It should be appreciated that the elevation 15 as wellas the concavity in FIG. 3c are comparatively small so that the loadapplication surface is still located substantially in a plane in thelongitudinal direction of the neutral axis of the measuring beam.

The load to be measured could for instance be a tank or a largecontainer which is affected by thermal expansions. The loading bracket 8is then secured to the supporting leg of the tank and arranged in avertical, natural working position along the load application surface 11and from which position it is prevented to deviate by means of itsbottom member 13. This bottom member prevents the loading bracket fromleaving the load cell even in case of lifting forces, due to forinstance heavy winds on an outdoor tank or container. In order to at allallow such a closed loading bracket the underside 16 of the outermostportion of the measuring beam section with reduced material thicknesshas a straight or rounded bevelled portion 17. Thanks to this bevelledportion the loading bracket can be put on to the beam under an angle andthen put into a vertical position in which the loading bracket has avertical play which is less than the height of the gliding stop 12.

The invention is not limited to the illustrated embodiments but can bevaried within the scope of the accompanying claims.

What is claimed is:
 1. A shear beam load cell, comprising:a shear beammember comprising a mounting end portion and a free deflectable endportion, the free deflectable end portion having an upper loadapplication side, a lower side, and side surfaces, at least one straingauge being mounted on the side surfaces so as to be affected bydeformation of the free deflectable end portion of the shear beammember; the free deflectable end portion including a section of reducedthickness on the upper side with a load application surface beinglocated thereon, the load application surface being arrangedsubstantially along a longitudinal neutral axis of the shear beam memberand having a longitudinal extension that permits a point-of-loadapplication of a load force to be moved along the load surface as aresult of thermal expansion in a weighing container or foundation. 2.The shear beam load cell according to claim 1, wherein the shear beammember further comprises a glide stop protection arranged at an end ofthe free deflectable end portion of the shear beam for limiting amagnitude of movement of the point of load application of the load forceon the load application surface.
 3. The shear beam load cell accordingto claim 1, wherein the load application surface has a rounded, convexprofile.
 4. The shear beam load cell according to claim 1, wherein theload application surface has a trapezoidal, convex profile.
 5. The shearbeam load cell according to claim 1, wherein the load applicationsurface has a rounded, concave profile.
 6. The shear beam load cellaccording to claim 1, wherein the load application surface has atrapezoidal, concave profile.
 7. The shear beam load cell according toclaim 1, further comprising:a U-shaped contact bracket for applying theload force to the free deflectable end portion of the shear beam, thecontact bracket comprising a contact surface between two legs, thecontact surface of the bracket engaging the load application surface ofthe shear beam.
 8. The shear beam load cell according to claim 7,wherein the loading bracket further comprises a bottom member providinga closed design and tiling stop protection to the bracket.
 9. The shearbeam load cell according to claim 8, wherein an underside of the sectionof reduced thickness of the free deflectable end portion includes anouter beveled portion for permitting placement of the loading bracketonto the beam at an angle before placement in a vertical loadapplication position.